The Collaborative International Dictionary of English v.0.48:

Rarefy \Rar"e*fy\ (r[a^]r"[-e]*f[imac]; 277), v. t. [imp. & p.
   p. Rarefied (r[a^]r"[-e]*f[imac]d); p. pr. & vb. n.
   Rarefying (r[a^]r"[-e]*f[imac]`[i^]ng).] [F. rar['e]fier;
   L. rarus rare + -ficare (in comp.) to make; cf. L.
   rarefacere. See -fy.]
   To make rare, thin, porous, or less dense; to expand or
   enlarge without adding any new portion of matter to; --
   opposed to condense.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rarefied
    adj 1: having low density; "rare gasses"; "lightheaded from the
           rarefied mountain air" [syn: rare, rarefied,
           rarified]
    2: of high moral or intellectual value; elevated in nature or
       style; "an exalted ideal"; "argue in terms of high-flown
       ideals"- Oliver Franks; "a noble and lofty concept"; "a grand
       purpose" [syn: exalted, elevated, sublime, grand,
       high-flown, high-minded, lofty, rarefied, rarified,
       idealistic, noble-minded]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "rarefied":
   adulterated, air-built, airy, attenuate, attenuated, boyish,
   chimerical, clannish, cliquish, cloud-built, cut, dainty, delicate,
   diaphanous, dilute, diluted, dreamlike, elevated, esoteric,
   ethereal, exalted, exclusive, fanciful, fatuitous, fatuous, fine,
   fine-drawn, finespun, flimsy, frail, gaseous, gauzy, girlish,
   gossamer, gossamery, gracile, high, illusory, imaginary,
   insubstantial, lacy, lean, light, lofty, misty, noble, papery,
   phantomlike, private, rare, reduced, scant, scanty, select,
   shadowy, slender, slenderish, slight, slight-made, slim, slimmish,
   slinky, small, sparse, spirituous, sublime, subtile, subtle,
   svelte, sylphlike, tenuous, thin, thin-bodied, thin-set, thin-spun,
   thinned, thinned-out, thinnish, threadlike, uncompact,
   uncompressed, unreal, unsubstantial, vague, vaporous, wasp-waisted,
   watered, watered-down, watery, weak, willowy, windy, wiredrawn,
   wispy